This invention relates to a water-injected screw compressor which comprises a compressor element with two rotors driven by a motor, which rotors are rotatable in an interior space of the compressor element, a suction conduit which connects to an inlet part, situated at the top of this interior space of the compressor element, with an inlet valve which can close off the exit giving out to this inlet part, a pressure conduit which connects to this interior space of the compressor element and in which a vessel, which at the same time is a water separator, is installed, whereby an injection conduit is installed between the vessel and the interior space of the compressor element, for the injection of water into this interior space, which injection conduit comprises a part which is situated higher than the upper side of the inlet part of the interior space of the compressor element.
With most of the oil-injected compressors, at the outlet of the compressor element or in the pressure conduit, a valve, mostly a return valve, is provided; a valve is also provided in the injection conduit.
The valve at the outlet prevents that, when the compressor element suddenly stops before the inlet valve is closed, compressed air from the vessel flows outward through the compressor element and the suction conduit.
The valve in the injection conduit prevents that, with a sudden stop, too much oil will flow through the injection conduit into the interior space of the compressor element and this latter would be filled by oil, which subsequently would prevent the starting of the compressor element, as oil is not compressible.
As the functioning of said valves, however, becomes unreliable in a watery environment, the valves at the outlet and in the injection conduit of water-injected compressors mostly are omitted and a special inlet valve is used in the suction conduit, to wit a so-called “unloader”.
This “unloader” is a controlled valve mechanism which closes off the inlet just before or immediately after switching off the compressor, such that no compressed gas from the compressor element can be blown towards outside through the suction conduit, and which prevents that, as a result of the pressure in the vessel, the compressor element should be filled with water through the injection conduit. Through this valve mechanism, after stopping, compressed gas can be blown off from the vessel.
When with such compressors, the inlet valve closes off the suction conduit before or immediately after the stop of the compressor element, the vessel and the compressor element are at the same pressure, in consideration of the fact that no return valve is present at the outlet of the compressor element.
At that moment, the water level in the vessel is situated between a minimum and a maximum, and if this water level is higher than the underside of the compressor element, then, as no valve is present in the injection conduit, water from the vessel will flow back to the compressor element, as a result of the suction effect, until the water level in the vessel is equal to the one in the compressor element.
The compressor element can become filled up with water in such a manner that starting becomes impossible.
In order to prevent this, the position of the vessel in respect to the compressor element is chosen such that the maximum water level in the vessel is situated at the height of the bottom side of the rotors in the compressor element.
This may cause problems for the construction of the compressor and has as a consequence that the compressor takes up relatively much space and therefore also the housing in which the vessel, the compressor element and the motor are installed, is relatively large.